A variety of systems are used in borehole geophysical exploration and production operations to determine chemical and physical parameters of materials in the borehole environs. The borehole environs include materials, such as fluids or formations, near a borehole as well as materials, such as fluids, within the borehole. The various systems include, but are not limited to, formation testers and borehole fluid analysis systems conveyed within the borehole. In all of these systems, it is preferred to make all measurements in real-time and within instrumentation in the borehole. However, methods that collect data and fluids for later retrieval and processing are not precluded.
Formation tester systems are used in the oil and gas industry primarily to measure pressure and other reservoir parameters of a formation penetrated by a borehole, and to collect and analyze fluids from the borehole environs to determine major constituents within the fluid. Formation testing systems are also used to determine a variety of properties of the formation or reservoir near the borehole. These formation or reservoir properties, combined with in situ or uphole analyses of physical and chemical properties of the formation fluid, can be used to predict and evaluate production prospects of reservoirs penetrated by the borehole. By definition, formation fluid refers to any and all fluid including any mixture of fluids.
Formation tester tools can be conveyed along the borehole by variety of means including, but not limited to, a single or multi-conductor wireline, a “slick” line, a drill string, a permanent completion string, or a string of coiled tubing. Formation tester tools may be designed for wireline usage or as part of a drill string. Tool response data and information as well as tool operational data can be transferred to and from the surface of the earth using wireline, coiled tubing and drill string telemetry systems. Alternately, tool response data and information can be stored in memory within the tool for subsequent retrieval at the surface of the earth.
Formation tester tools typically comprise a fluid flow line cooperating with a pump to draw fluid into the formation tester tool for analysis, sampling, and optionally for subsequent exhausting the fluid into the borehole. Typically, a sampling pad is pressed against the wall of the borehole. A probe port or “snorkel” is extended from the center of the pad and through any mudcake to make contact with formation material. The snorkel and pad are designed to isolate the pressure and fluid movement to and from the formation and the wellbore. The best sample to be analyzed and/or taken should be from an undisturbed formation without any wellbore contamination.
Fluid is drawn into the formation tester tool via a flow line cooperating with the snorkel. Fluid is sampled for subsequent retrieval at the surface of the earth, or alternately exhausted to the borehole via the flow lines and pump systems.
When performing formation tester probe operations in a wellbore, it is critical to maintain a proper seal against the formation while performing a drawdown/build-up sequence. As significant differential pressures (1,000's of psi) can be created during this operation, the sampling pad, typically made of an elastomeric material, may extrude between the surface of the wellbore and the interface of the snorkel. Generally, soft pliable rubber is wanted for the pad seal, however, this is more likely to extrude.